A typical conventional method of producing a thin film multilayer wiring board will be described with reference to steps (a) to (g) of the method as shown in FIGS. 1A-1C.
In the step (a), a metallic undercoat film 2 is formed on an upper surface of a substrate 1. This metallic undercoat film 2 is used as a plating electrode at a later stage.
In the step (b), a resist layer 3 is formed, and this layer is grooved into a configuration corresponding to a predetermined conductor pattern.
In the step (c), a conductor is filled in grooves 4 in the resist layer 3 by electroplating, thereby forming conductors 5 such as horizontally-extending wiring conductors, via hole conductors, through hole conductors and ground layer conductors.
In the step (d), the resist layer 3 is removed to expose the conductors 5.
In the step (e), the metallic undercoat film 2, except for those portions thereof disposed immediately beneath the conductors 5, is removed by etching.
In the step (f), an insoluble insulating layer 6 is formed on the upper surface of the substrate 1 so as to cover the conductors 5.
In the step (g), the insoluble insulating layer 6 is ground and polished so as to expose the upper surfaces of the conductors 5 and also to flatten the upper surface of the insoluble insulating layer 6.
With these steps (a) to (g), a thin film single-layer wiring board is formed, and then these steps (a) to (g) are repeated several times to form a thin film multilayer wiring board.
This conventional method is described in "Thin-film Conductors and Conductor Processes" on pages 710 to 714 (Item 9.6.2) of Microelectronics Packaging, edited by Rao R, Tummala and Eugene J. Rymaszewski, VAN NOSTRAND REINHOLD, and is described in detail particularly in FIGS. 9-21 on page 713.
In the above conventional method, however, when a thin film multilayer wiring board is to be produced, it is necessary to carry out the steps (a) to (g) illustrated in FIGS. 1A to 1G for forming each of the layers of the multilayer wiring board, and therefore the number of the steps involved is very large. Particularly, in the step (f), the insoluble insulating layer 6 is formed on the rugged portion defined by the substrate 1 and the conductors 5, and therefore the insoluble insulating layer 6 thus formed also becomes rugged. As a result, the step (g) of grinding and polishing the insoluble insulating layer 6 so as to flatten this layer is essential.
As later described, as the insoluble insulating layer, a polymeric material, such as polyimide, polyamide, polyamide-imide, polyester and polycarbonate may be employed. These materials become softened and viscous due to a temperature rise during the grinding and polishing operation, which results in a problem that the grinding and polishing are difficult and take much time. Another problem is that when the via conductors and so on are ground simultaneously with the grinding of the insoluble insulating layer, metal powder resulting therefrom is scattered, and adheres to the softened polymeric material. The metal powder may not be easily removed from the polymeric material by washing.
Therefore, there is a possibility that contaminants, such as powder of the polymer, may be present in each layer of the thin film multilayer wiring board. Therefore, in order to enhance the reliability, the step of washing away the contanimants, as well as the step of inspecting an electrical defect, are needed in addition to the above steps (a) to (g). Moreover, when an electrically-defective product (the thin film multilayer wiring board having a defect at least in one layer thereof) is found in this inspection step, an additional step must be carried out in order to reuse such an electrically-defective product, and it is practically difficult to remove the several .mu.m-thick thin film layer at the defective portion. Consequently, a considerable number of defective products have been produced, and the yield rate was inevitably low.
Further, in the thin film multilayer wiring board produced by the above method, the insoluble insulating layer 6 is coated on the rugged portion defined by the substrate 1 and the conductors 5, and therefore a liquid material must be used for forming the insoluble insulating layer 6. When the liquid material is to be cured or set into a solid by a heat treatment, stresses of different values develop due to a large difference in thermal expansion coefficient between the substrate 1, the metallic undercoat film 2, the conductors 5 and etc. These stresses have caused warping of the thin film multilayer wiring board.
As described above, the above conventional method of producing the thin film multilayer wiring board suffers from the problems with the mass productivity, the reliability and the yield rate, and besides there is encountered another problem that warping is present in the resultant thin film multilayer wiring board.